In a display device using a self-light emitting type light emitting element represented by an organic light emitting diode (also referred to as an OLED (Organic Light Emitting Diode), an organic EL element, an electroluminescence (EL) element and the like), a passive matrix method and an active matrix method are known as its driving method. The former has a simple structure, but has a problem such that a realization of a large and high definition display is difficult. Therefore, the active matrix method is actively developed in recent years in which a current flowing to the light emitting element is controlled by a thin film transistor (TFT) provided in a pixel circuit.
In the case of a display device of the active matrix method, there is a problem that a current flowing to a light emitting element varies due to a variation in current characteristics of driving TFTs, which varies a luminance. That is, a driving TFT which drives a current flowing to the light emitting element is used in a pixel circuit. When characteristics of these driving TFTs vary, a current flowing to the light emitting element varies, which varies a luminance. Then, various circuits to suppress a variation in luminance are suggested in which a current flowing to a light emitting element does not vary even when characteristics of driving TFTs in a pixel circuit vary. (For example, see Patent Documents 1 to 4)    [Patent Document 1]    Published Japanese Translation of PCT International Publication for Patent Application No. 2002-517806    [Patent Document 2]    International Publication WO01/06484    [Patent Document 3]    Published Japanese Translation of PCT International Publication for Patent Application No. 2002-514320    [Patent Document 4]    International Publication WO02/39420
Patent Documents 1 to 3 disclose a circuit configuration for preventing a variation of a current value flowing to a light emitting element due to a variation in characteristics of driving TFTs arranged in a pixel circuit. This configuration is referred to as a current write type pixel or a current input type pixel. Patent Document 4 discloses a circuit configuration for suppressing a variation of a signal current due to a variation of TFTs in a source driver circuit.
FIG. 6 shows a first configuration example of a conventional active matrix type display device disclosed in Patent Document 1. The pixel shown in FIG. 6 comprises a source signal line 601, first to third gate signal lines 602 to 604, a current supply line 605, TFTs 606 to 609, a capacitor 610, an EL element 611, and a current source 612 for inputting a signal current.
An operation from a write of a signal current to a light emission is described with reference to FIG. 7. Reference numerals denoting each portion in the drawing correspond to those in FIG. 6. FIGS. 7A to 7C each schematically shows a current flow. FIG. 7D shows a relationship of a current flowing each path when writing a signal current. FIG. 7E shows a voltage accumulated in the capacitor 610 when writing a signal current, that is a gate-source voltage of the TFT 608.
First, a pulse is inputted to the first gate signal line 602 and the second gate signal line 603 and the TFTs 606 and 607 are turned ON. At this time, a current flowing through the source signal line, that is a signal current is denoted as Idata.
As the current Idata flows through the source signal line, the current path is divided into I1 and I2 in the pixel as shown in FIG. 7A. These relationships are shown in FIG. 7D. It is needless to say that Idata=I1+I2 is satisfied.
A charge is not held in the capacitor 610 at the moment the TFT 606 is turned ON, therefore, the TFT 608 is OFF. Therefore, I2=0 and Idata=I1 are satisfied. In other words, current only flows into the capacitor 610 to be accumulated in the meantime.
After that, as the charge is gradually accumulated in the capacitor 610, a potential difference starts to generate between both electrodes (FIG. 7E). When the potential difference between the both electrodes reaches Vth (a point A in FIG. 7E), the TFT 608 is turned ON and I2 generates. As described above, as Idata=I1+I2 is satisfied, current still flows and a charge is accumulated in the capacitor while I1 decreases gradually.
The charge keeps being accumulated in the capacitor 610 until the potential difference between the both electrodes, that is a gate-source voltage of the TFT 608 reaches a desired voltage, that is a voltage (VGS) which can make the TFT 608 flow the current Idata. When the charge stops being accumulated (a point B in FIG. 7E), the current I1 stops flowing and the TFT 608 flows a current corresponding to VGS at that time, thus Idata=I2 is satisfied (FIG. 7B). Thus, a write operation of a signal is terminated. At last, selections of the first gate signal line 602 and the second gate signal line 603 are terminated to turn OFF the TFTs 606 and 607.
Subsequently, a light emitting operation starts. A pulse is inputted to the third gate signal line 604 to turn ON the TFT 609. As the capacitor 610 holds VGS which is written before, the TFT 608 is ON and the current Idata flows from the current supply line 605. Thus, the EL element 611 emits light. Provided that the TFT 608 is set to operate in a saturation region, Idata keeps flowing without change even when a source-drain voltage of the TFT 608 changes.
In this manner, an operation to output a set current is hereinafter referred to as an output operation. As a merit of the current write type pixel, a desired current can be accurately supplied to an EL element because a gate-source voltage required to flow the current Idata is held in the capacitor 610 even when the TFTs 608 have a variation in characteristics and the like. Therefore, a luminance variation due to the variation in characteristics of TFTs can be suppressed.
The aforementioned examples relate to a technology for correcting a change of current due to a variation of driving TFTs in a pixel circuit, however, the same problem occurs in a source driver circuit as well. Patent Document 4 discloses a circuit configuration for preventing a change of a signal current due to a variation of the TFTs in the source driver circuit generated in fabrication.
Further, there is a driver circuit of a light emitting element provided with a current supply circuit (1) and a drive control circuit (2a) which have configurations that are capable of leading a current (Is) having the same current value as a current (Ir) flowing from a supply transistor (M5) which supplies a current to drive a light emitting element (EL) to a drive control circuit (2a) through a reference transistor (M4), and of controlling so that the current (Is) approaches a desired set current value (Idrv) and each source-drain voltage data (Vs, Vr) become equal to each other based on the current (Is), the source-drain voltage data (Vs) of the reference transistor (M4) and source-drain voltage data (Vr, Vdrv) of the supply transistor (M5). (see Patent Document 5)    [Patent Document 5]    Published Japanese Translation of PCT International Publication for Patent Application No. 2003-108069 (p. 5 to 6, FIG. 6)
Further, there is a known technology configured with a light emitting element provided in series between a first power source and a second power source, a driving transistor which drives the light emitting element, a first switching transistor for leading a control signal for controlling the driving transistor to a gate of the driving transistor, a differential amplifier for comparing a voltage at a connection node of the light emitting element and the driving transistor and a control voltage which shows a luminance of a pixel, which is inputted to the display device, and configured so that the control signal is lead to the gate of the driving transistor through the first switching transistor. (see Patent Document 6)    [Patent Document 6]    Published Japanese Translation of PCT International Publication for Patent Application No. 2003-58106 (p. 3 to 4, FIG. 1)
In this manner, in a conventional technology, a signal current and a current for driving a TFT, or a signal current and a current which flows to a light emitting element when it emits light are configured to be equal or in proportion to each other.